Fast alpha transparency rendering method

• US 5,923,333 A
• Filed: 01/06/1997
• Issued: 07/13/1999
• Est. Priority Date: 01/06/1997
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Stroyan, Howard D.
• Primary Examiner(s): Nguyen, Phu K.
• Application Number:
  US08/778,858
• Time in Patent Office: 918 Days
• Field of Search: 345/422, 345/421, 345/419, 345/113, 345/114
• US Class Current: 345/422
• CPC Class Codes: G06T 15/405 using Z-buffer

Abstract

A fast method for rendering opaque and transparent objects that produces a higher quality image at a greater speed for a given level of hardware support. Opaque objects are rendered first utilizing the z-buffer as a solids only depth buffer. Transparent objects are then rendered in multiple passes. Transparent objects are processed in a back to front order to eliminate surface anomalies. The z-buffer is utilized in an alternative mode so that the front-most surfaces of objects in a scene are processed last. Back-facing primitives of transparent objects are rendered and alpha blended first and then the front-facing primitives are rendered and alpha blended second.

Claims

1. A method for rendering opaque objects and transparent objects in at least one portion of a scene for graphic display in a computer system, said method comprising the steps of:

(a) rasterizing primitives of said opaque objects and sending depth values for pixels of said rasterized primitives to a z-buffer and sending image data for said pixels of said rasterized primitives to a frame buffer in said computer system;

(b) processing primitives of said transparent objects, said processing comprising the steps of

(b1) disabling writes to said z-buffer,

(b2) culling all front-facing primitives of said transparent objects,

(b3) rasterizing all back-facing primitives of said transparent objects to yield back-facing primitive pixels, and culling all said back-facing primitive pixels when depth values of said back-facing primitive pixels indicate that said back-facing primitive pixels are located behind at least one of said opaque objects, and

(b4) alpha blending image data of remaining back-facing primitives with said image data in said frame buffer, and sending said alpha blended image data of said remaining back-facing primitives to said frame buffer, and

(c) processing said primitives of said transparent objects, said processing comprising the steps of

(c1) disabling writes to said z-buffer,

(c2) culling said back-facing primitives of said transparent objects,

(c3) rasterizing all front-facing primitives of said transparent objects to yield front-facing primitive pixels, and culling all said front-facing primitive pixels when depth values of said front-facing primitive pixels indicate that said front-facing primitive pixels are located behind at least one of said opaque objects, and

(c4) alpha blending image data of remaining front-facing primitives with said image data in said frame buffer, and sending said alpha blended image data of said remaining front-facing primitives to said frame buffer.

2. A method for rendering transparent objects in at least one portion of a scene for graphic display in a computer system, said method comprising the steps of:

(a) processing primitives of said transparent objects, said processing comprising the steps of

(a1) disabling writes to a z-buffer,

(a2) culling all front-facing primitives of said transparent objects, and

(a3) rasterizing back-facing primitives of said transparent objects to yield back-facing primitive pixels and alpha blending image data of said rasterized back-facing primitive pixels of said transparent objects and sending said alpha blended image data of said rasterized back-facing primitive pixels to a frame buffer; and

(b) processing said primitives of said transparent objects, said processing comprising the steps of

(b1) disabling writes to said z-buffer,

(b2) culling all back-facing primitives of said transparent objects, and

(b3) rasterizing front-facing primitives of said transparent objects to yield front-facing primitive pixels and alpha blending image data of said rasterized front-facing primitive pixels of said transparent objects and sending said alpha blended image data of said rasterized front-facing primitive pixels to said frame buffer.

3. A method for rendering transparent objects in at least one portion of a scene for graphic display in a computer system, said method comprising the steps of:

(a) creating a shell of front-most primitives of said transparent objects by rasterizing said transparent objects to establish a depth image of said front-most primitives of said transparent objects in a z-buffer, said rasterizing comprising determining z-values of pixels of said front-most primitives of said transparent objects, storing said z-values of said pixels in said z-buffer, and discarding all frame buffer data;

(b) disabling writes to said z-buffer, rasterizing said transparent objects, and alpha blending image data of pixels of said rasterized transparent objects when z-values of said pixels are not equal to said z-values stored in said z-buffer; and

(c) disabling writes to said z-buffer, rasterizing said transparent objects, and alpha blending image data of pixels of said rasterized transparent objects when z-values of said pixels equal said z-values stored in said z-buffer.

4. The method of claim 3 further comprising performing the following steps (b1) and (b2) in place of step (b):

(b1) disabling writes to said z-buffer, rasterizing all back-facing primitives of said transparent objects, and alpha blending image data of pixels of said rasterized back-facing primitives when z-values of said pixels are not equal to said z-values stored in said z-buffer; and

(b2) disabling writes to said z-buffer, rasterizing all front-facing primitives of said transparent objects, and alpha blending image data of pixels of said rasterized front-facing primitives when said z-values of said pixels are not equal to said z-values stored in said z-buffer.